The hip joint is a ball-and-socket type joint in which the ball-shaped femoral head is engaged with and articulates with a cup-shaped socket known as the acetabulum. Injury or disease may damage the hip joint to the extent that it must be replaced by or augmented with a prosthetic joint. Deterioration of the acetabulum, and particularly the cartilage within the acetabulum, requires that a prosthetic acetabular shell be mounted within a prepared area of the acetabulum. The acetabular shell receives and articulates with a prosthetic femoral head which extends from a femoral stem that is installed within a proximal portion of a patient's femur.
A successful hip replacement or arthroplasty procedure results, in part, from selection of prosthetic joint components that are dimensioned and positioned to closely approximate the geometry and functional characteristics of a natural, healthy hip joint. A successful arthroplasty procedure also requires a strong attachment between the prosthetic devices and the patient's bone at the time of implantation and throughout the life of the prosthesis. Successful attachment of the acetabular shell to the patient's acetabulum is critical to a successful arthroplasty procedure.
Generally, acetabular shells are hemispherical cups that are secured within a patient's prepared acetabulum by an interference fit, mechanical attachment devices or adhesives such as bone cement. Bone cement provides good attachment qualities upon implantation of the acetabular shell, but the attachment can deteriorate over time. Under repeated loading, the cement can fatigue and fracture, resulting in loosening of the prosthesis and the formation of debris within and around the prosthesis. Loosening of the prosthesis is obviously problematic because removal and replacement of a loosened acetabular shell may result in the loss of a great deal of natural bone as the cement is removed from the patient's acetabulum.
Mechanical fixation of acetabular shells, using screws or similar mechanical fixation devices, is often effective but not always preferred. Factors such as the availability of bone of sufficient strength and quality and the proximity of arteries limit the application and effectiveness of the mechanical fixation devices.
Conventional methods for implanting acetabular shells also include the use of an interference fit between the prosthetic acetabular shell and bone. One such method involves reaming a spherical cavity in the patient's acetabulum and forcibly inserting therein a hemispherical acetabular shell having a radius that is greater than that of the reamed cavity. As a result of the greater size of the acetabular shell, an interference fit is created between acetabular shell and the patient's acetabulum. Generally, such an oversized acetabular shell is retained within the acetabulum by horizontal forces applied about the peripheral edge of the shell by interference with the smaller acetabular opening. However, vertical reactive forces are also applied in the apical region of the hemispherical shell. These vertical forces can make it difficult to optimally seat and retain the shell within the acetabulum.
Other known acetabular shells employ a so-called "dual radius" or "dual geometry." One such device, described in U.S. Pat. No. 4,704,127, is an acetabular shell that has a hemispherical shape with a frustro-conical surface portion which protrudes radially outward in proximity to the periphery of the hemisphere, providing the shell with a stepped exterior surface. The acetabulum is prepared by reaming to create a spherical portion having the same radius as the spherical portion of the shell. Separately, a frustro-conical portion is reamed having a smaller radius than the frustro-conical surface portion of the shell. When the shell is inserted into the prepared acetabulum, an interference fit is created at the interface of the differently sized frustro-conical surface portions. While this embodiment eases some of the disadvantages of the prior methods, the two-step reaming process increases the complexity of the operation and instrumentation and may result in a less precise fit. This process also requires removal of additional amounts of the patient's bone. For obvious reasons, it is preferred to remove less rather than more of the healthy portions of the patient's bone when implanting orthopedic prostheses.
A further example of an acetabular shell adapted for interference fixation is provided in U.S. Pat. No. 4,892,549. This acetabular shell is generally hemispherical, having an apical spherical region with a first radius and a peripheral spherical region with a second, larger radius resulting in a stepped outer surface. Such an acetabular shell is implanted within an acetabulum that is prepared by reaming spherically at a radius approximately equal to the radius of the apical spherical region of the shell. An interference fit is thus created at the interface of the larger peripheral spherical region of the shell with the acetabulum.
Each of the acetabular shells described above requires a unique prosthesis part which must be kept in inventory in a variety of sizes. Furthermore, this inventory must be kept in proximity to the operating environment to allow the orthopedic surgeon to select the best implantation technique depending on factors such as bone consistency and quality that are revealed only during implantation surgery.
Accordingly, there is need for acetabular instrumentation that removes smaller amounts of the patient's bone, provides for a high quality interference fit with the portions of the acetabulum where the bone is strongest, and allows surgeons the option to vary the amount of interference and the method of implantation during surgery without increasing prosthesis inventory.